Are the note contents completely gone, or are the null characters simply added on top of them?
This may a wrong clue, but there was a somewhat alike case with Joplin, which is also a note taking application (see https://forum.syncthing.net/t/error-syncing-joplin-notes-help-configuring-syncthing/14158). That problem turned out to be related to Joplin and some Android versions.
Is Android involved here too? I’m guessing that “my Samsung phone” is Android, right? You may want to report the problem to Obsidian as well, since Syncthing itself doesn’t do anything special to markdown files (or any other format, for that matter).